In a number of applications, there is a need to provide a constant source of power so that critical equipment will not be adversely effected by power interruptions. Such critical equipment includes microprocessors and electronic memories having large quantities of stored information that is lost when power is temporarily removed. This type of storage is often labeled "semi-permanent" or "volatile," in contrast with the permanent memory available with magnetic core technology. Modern telephone equipment includes the use of volatile memory devices to store small quantities of information such as telephone numbers. Even here, the annoyance of having to re-enter data after a power interruption is unacceptable.
A number of techniques exist for switching from one voltage source to another when the primary voltage source falls below a predetermined threshold. One prior art technique is disclosed in U.S. Pat. No. 4,122,359 entitled Memory Protection Arrangement and issued to Breikss. In this patent, diodes are used to connect a pair of power supplies to a load--one of the power supplies being a battery. This technique not only incurs a significant voltage drop penalty across the diode, but allows both power supplies to be on simultaneously under certain conditions. Another technique is disclosed in U.S. Pat. No. 3,557,003 entitled Automatic Battery Switching Device, and issued to Behr et al. In this patent, bipolar transistors are used to transfer power between working and standby batteries when the voltage level of the presently operating battery falls below a threshold level. Associated with this configuration are bias and leakage currents that may adversely effect the selected battery.
In applications utilizing low power metal oxide semiconductor (MOS) memory devices and/or microprocessors, the use of a substantially permanent lithium battery (i.e., soldered onto a circuit board) is highly desirable for both cost and size considerations. Permanently mounted lithium batteries present certain problems, however, not solved by the above-identified patents. For example, lithium batteries can tolerate only minimum reverse bias current before damage results. Further, the useful life of non-rechargeable batteries is directly related to leakage current when the battery is not in use.
In a telephone application, where power is derived from the telephone line, only a minimum voltage may be available and it is imperative that the power supply switching apparatus cause little or no voltage drop to the load or require bias currents of a magnitude that would measurably reduce the available voltage. The mere replacement of bipolar transistors, requiring significant bias current, with high impedance MOS transistors, requiring virtually none, brings along additional properties that heretofore have largely been ignored in connection with power supply switching arrangements. One such property is the presence of a fully bilateral switch in connection with a battery that cannot tolerate reverse currents.
It is therefore an object of the present invention to provide a power supply switching arrangement having minimum voltage drop that maximizes the useful life of a secondary battery, such as one of lithium construction, through substantial elimination of leakage and reverse bias currents.
It is another object of this invention to provide an arrangement of minimum size and cost that may be readily incorporated in the monolithic construction of MOS devices.